1. Field of the Invention
The present invention relates generally to a W-CDMA (Wide-band Code Division Multiple Access) mobile communication system, and in particular, to a transmitting/receiving apparatus and method for increasing decoding performance at retransmission.
2. Description of the Related Art
Adverse influences on high-speed, high-quality data service are attributed to a channel environment in a mobile communication system. The radio channel environment varies frequently because of signal power changes caused by white noise and fading, shadowing, the Doppler effect that occurs due to the movement and frequent velocity change of a terminal, and interference from other users and multi-path signals. Therefore, aside from conventional technologies in the second or third generation mobile communication system, an advanced technique is required to support wireless high-speed data packet service. In this context, the 3GPP (3rd Generation Partnership Project) and the 3GPP2 commonly address the techniques of AMCS (Adaptive Modulation & Coding Scheme) and H-ARQ (Hybrid Automatic Repeat Request).
The AMCS adjusts a modulation order and a code rate according to changes in a downlink channel condition. The downlink channel quality is usually obtained by measuring an SNR (Signal-to-Noise Ratio) of a received signal at a UE (User Equipment). The UE transmits the channel quality information to a BS (Base Station) on an uplink. Then the BS estimates the downlink channel condition based on the channel quality information and determines an appropriate modulation scheme and code rate for a channel encoder according to the estimated downlink channel condition.
Implementation of H-ARQ is challenging because there are many considerations in terms of system complexity including reception buffer size and signaling as well as channel quality.
QPSK (Quadrature Phase Shift Keying), 8PSK (8-ary PSK), and 16QAM (16-ary Quadrature Amplitude Modulation) and code rates of 1/2 and 1/4 are used in the existing high-speed wireless data packet communication system. In AMCS, a BS applies a high-order modulation (e.g., 16QAM and 64QAM) and a high code rate of 3/4 to a UE having good channel quality such as its adjacent UEs, and a low-order modulation (e.g., 8PSK and QPSK) and a low code rate of 1/2 to a UE having bad channel quality such as a UE at a cell boundary. The AMCS significantly reduces interference signals and improves system performance, as compared to the conventional method relying on high-speed power control.
The H-ARQ is a retransmission control technique used to compensate for errors in initially transmitted data packets. The H-ARQ is divided into chase combining (CC), full incremental redundancy (FIR), and partial incremental redundancy (PIR).
The CC is retransmission of the same packet as used at initial transmission. A receiver combines the retransmission packet with the initial transmission packet stored in a reception buffer, thereby increasing the reliability of coded bits input to a decoder and thus obtaining the performance gain of the overall mobile communication system. About a 3-dB performance gain is affected on the average since combining the same two packets is equivalent to repeated coding of the packet.
The FIR is retransmission of a packet having only parity bits. A decoder decodes data using the new parity bits as well as initially transmitted systematic bits. As a result, decoding performance is increased. It is well known in coding theory that a higher performance gain is obtained at a low code rate than by repeated coding. Therefore, the FIR is superior to the CC in terms of performance gain.
The PIR is retransmission of a packet that is a combination of systematic bits and new parity bits. A receiver combines the retransmitted systematic bits with initially transmitted systematic bits during decoding. Thus, the PIR is similar to the CC in effect. The PIR is also similar to the FIR in that the new parity bits are used at decoding. Since a relatively high code rate is used in the PIR than in the FIR, the PIR is in the middle of the FIR and the CC in terms of performance.
Consequently, use of the independent techniques of AMCS and H-ARQ can improve system performance significantly.
FIG. 1 is a block diagram of a transmitter in a typical high-speed wireless data packet communication system. Referring to FIG. 1, the transmitter includes a channel encoder 110, a rate controller 120, an interleaver 130, a modulator 140, and a controller 150. Upon input of information bits in transport blocks of size N, the channel encoder 110 encodes the information bits at a predetermined code rate (e.g., 1/2 or 3/4). The channel encoder 110 can support a plurality of code rates using a mother code rate of 1/6 or 1/5 through symbol puncturing or symbol repetition. The controller 150 controls the code rate.
The rate controller 120 matches the data rate of the coded bits generally by transport channel-multiplexing, or by repetition and puncturing if the number of the coded bits is different from that of bits transmitted in the air. To minimize data loss caused by burst errors, the interleaver 130 interleaves the rate-matched bits. The modulator 140 modulates the interleaved bits in a modulation scheme determined by the controller 150.
The controller 150 controls the code rate of the channel encoder 110 and the modulation scheme of the modulator 140 according to the radio downlink channel condition. To selectively use QPSK, 8PSK, 16QAM, and 64QAM according to a radio environment, the controller 150 supports AMCS. Though not shown, a UE spreads the modulated data with a plurality of Walsh codes to identify data transport channels and with a PN code to identify a BS.
FIG. 2 illustrates a signal constellation when using 16QAM in the modulator 140. Referring to FIG. 2, 4 coded bits form a single modulation symbol, which is mapped to one of 16 signal points. The 16 modulation symbols are categorized into region 1 having the highest error probability, region 2 with an intermediate error probability, and region 3 with the lowest error probability according to how easily a receiver can identify corresponding modulation symbols.
FIG. 3 is a graph illustrating error probabilities of the regions in a simulation under an AWGN (Additive White Gaussian Noise) environment. For example, modulation symbols 6, 7, 10, and 11 in region 1 have a relatively high error probability.
Modulation symbols in a high-order modulation scheme such as 16QAM or 64QAM can be grouped into regions having different error probabilities. In this case, retransmission bits have the same error probability as initial transmission bits in conventional H-ARQ. Consequently, error probability is identical in initial transmission and retransmission.
It is known in the art that decoding performance is improved when an LLR (Log Likelihood Ratio) of input bits is uniform. Yet transmission of particular bits with a high error probability deteriorates the decoding performance. Thus there is a need for a novel retransmission technique that increases performance at retransmission.